An image forming apparatus provided with a fixing device is well-known. The image forming apparatus forms an image on a medium such as paper or a plastic sheet with a coloring material such as ink or toner based on image information. Then the fixing device applies heat and pressure to the image on the medium to fix the image on the medium. The medium on which the image is fixed is discharged to a given position.
There are various types of fixing devices. In recent years, a type of fixing device has been used which includes a pressing roller and an endless fixing belt capable of generating heat through electromagnetic induction. Such a fixing belt is supported to be capable of moving annularly on a pressing roller. The fixing belt basically includes an inner substrate layer, a heat generation layer laminated around the outer peripheral surface of the substrate layer to generate heat through electromagnetic induction, and an elastic layer laminated around the outer peripheral surface of the heat generation layer.
A pressure pad is arranged at a part of the inner peripheral surface of the fixing belt adjacent to the pressing roller. The pressure pad presses the part of the outer peripheral surface of the fixing belt adjacent to the pressing roller against the outer peripheral surface of the pressing roller. An electromagnetic induction heating device is arranged adjacent to the outer peripheral surface of the fixing belt at a position far away from the pressing roller. A magnetic shunt member and a magnetic shielding member are arranged adjacent to the inner peripheral surface of the fixing belt at a position far away from the pressing roller.
In the electromagnetic induction heating type fixing device, the electromagnetic induction heating device heats the heat generation layer of the fixing belt at a given temperature, and the pressing roller and the fixing belt are rotated in the same given direction at the tangent. During this period, the medium on which an image is formed enters the tangent between the pressing roller and the fixing belt. The pressing roller and the fixing belt nip the medium with an image entering the space therebetween and convey the medium in the given direction. The heated fixing belt heats the image on the medium while pressing against the medium to fix the image on the medium.
It is preferred that the temperature along the tangent is uniform so that the fixing belt used in the electromagnetic induction heating type fixing device can uniformly fix the entire image on the medium. Therefore, the thickness of the heat generation layer of the fixing belt needs to be made uniform in the whole fixing belt.
However, according to the current technology, it is difficult to make the thickness of the heat generation layer of the fixing belt uniform in the whole fixing belt.
Thus, at the present time, a heating test is carried out on each of a plurality of fixing belts which has just been manufactured through a tester accompanied with the electromagnetic induction heating device before incorporating the fixing belt in the fixing device. In the heating test, the fixing belt that generates heat at a temperature beyond a given allowable temperature range is discarded.
The heating test makes the manufacture process of the fixing belt much more complicated, and causes an increase in the manufacture cost of the fixing belt. The discarded fixing belt also causes an increase in the manufacture cost of the fixing belt.